1. Field of the Invention
This invention relates to a vertical heat treatment apparatus used for manufacturing semiconductors.
2. Description of the Related Art
As a higher density has been required for semiconductor elements, more accurate heat treatment has come to be required for semiconductor devices. Wafers heat-treated in a reaction vessel in a heat treatment furnace such as a vertical or horizontal furnace is unloaded outside of a heat treatment path. When the reaction tube is uncovered soon after the heat treatment has been carried out, convention occurs in the reaction tube due to the temperature difference between the interior of the reaction tube and the outside thereof. It is necessary to keep the wafer atmosphere at a high temperature clean in order to increase the yield of wafers which are heat-treated accurately.
Japanese Laid-open Patent Application Publications No. 61-111524 and No. 62-146265 and Japanese Laid-open Utility Model Publication No. 62-8633 disclose apparatuses for making clean air flow sidewise onto wafers unloaded from a vertical heat treatment furnace. Japanese Laid-open Patent Application Publication No. 62-36817 discloses an apparatus for making air flow downward onto wafers unloaded from a horizontal heat treatment furnace. Further, Japanese laid-open Utility Model Application No. 64-26833 discloses an apparatus for making air side-flow at the unloading position of a horizontal heat treatment furnace.
In those conventional apparatuses, air supplied by a motor fan or the like is cleaned by passing through a filter and is made to flow onto wafers.
The following must be considered when clean air is supplied to the unloading position by means of a fan. When air to be supplied is circulated in a room, the temperature in the room is raised. In order to avoid this, air must be introduced into the room from another room by means of a fan. Since the unloading position is provided in a so-called maintenance room, pressure difference is provided between the maintenance room and the surrounding clean room such that the pressure in the clean room is positive. When, therefore, air in the utility zone is introduced into the clean room by the fan, an excessive load is exerted on the fan due to the pressure difference. Because the pressure difference is varied according to user's preference, the amount of air sent by a fan having the same blowing capacity changes and air cannot be sent in some cases.